The present invention relates to a system for the continuous mass polymerization of SAN (styrenic-alkenylnitrile) copolymer, and more especially to both a process and an apparatus for continuous mass polymerization of SAN.
The copolymerization of vinyl monomers, particularly styrene and acrylonitrile as applied here, is well known, as are the processes for their production. However, there are still many inherent problems which have either been unsolved, or in many cases, solved unsatisfactorily.
In the case of SAN polymerization, it is known that precise control is essential to the production of a product having acceptable properties. For example, if the acrylonitrile content of copolymer in a single product varies by more than about 4%, the different copolymers become incompatible with one another, resulting in an unacceptable, hazy product. Because styrene and acrylonitrile monomers polymerize at different rates, careful control is needed not only during the polymerization stage, but also during the subsequent purification stages. Especially in these latter processing stages, it is essential to maintain uniformity of the product, because even small amounts of copolymer product having a high acrylonitrile content can cause a yellow discoloration of the entire product, due to cyclization of adjacent pendant acrylonitrile groups upon heating of the copolymer, e.g., even during subsequent thermoforming steps.
This is an important factor in the post-polymerization treatment of the product in a SAN polymerization process, e.g., the removal of residual monomer from the polymer, known as devolatilizing the polymer. Once the polymer/monomer mixture leaves the reactor there is great risk of producing high-acrylonitrile-containing polymer, due to the uneven rates of polymerization for the two different monomer species and the high temperatures which are utilized for devolatilization. In conventional processes, devolatilization is typically carried out with thin film devolatilizing equipment, such as the so-called "Film Truder", which evaporates the liquid monomer very rapidly to minimize further polymerization. This thin filmm equipment, however, is relatively expensive and requires an inordinate amount of maintenance, e.g., at least once daily, because of its many moving parts and the extensive seals characteristic thereof. This maintenance requires, therefore, that the polymerization line be shut down or that some measure be taken to hold material upstream during maintenance. The result is an uneven quality of product and added expense for equipment and operation.